A manufacturing process for the production of absorbent products such as diapers, incontinence pads, sanitary towels and panty liners normally comprises a processing of various continuous webs of material, which are fed out from rolls or the like and passed through various work stations for the carrying out of various work elements and process steps. For example, it is common for a manufacturing process for absorbent products to provide a first web of material that defines a backing layer consisting of a plastic film that is non-permeable to liquid and a second web of material that defines an outer layer consisting of a liquid-permeable material, for example a non-woven material. The product can also be provided with other components, such as, for example, an absorbent core of a material intended to absorb bodily fluids.
The work elements that are carried out during a process of the type described above can consist, for example, of attaching together two or more layers of material, perforating, cutting, gluing, embossing a pattern or other type of shaping and processing of the materials involved. Other examples of work elements are the application of different components, such as fastening devices (so called tabs), cellulose centres, elastic material, so-called disposal tapes, etc.
In all, the webs of material in question go through various process steps that result in a continuous complete web of material that consists of a continuous row or strip of a number of absorbent products. Each individual product is finally shaped by this web being cut at regular intervals that correspond to the length of the finished product.
In a process of the type described above, some form of decorative element is often applied, such as a printed pattern or pictures, which are intended to enhance the visual impression of the finished product. Such a printing process is preferably carried out by conventional multi-colour printing. In particular, concerning absorbent products in the form of diapers for babies, such printed motifs, for example in the form of fairy-tale characters and cartoon characters, are considered to make the product more appealing to the consumer. In addition, such a procedure for printing a motif is suitably carried out on the backing layer for a diaper, not least due to the fact that such a backing layer is normally made of a polymer film that is essentially non-permeable to liquid, the surface of which is suitable for colour printing with a good quality and high resolution. In this way, a printed back is obtained on the finished product.
Certain types of printed motif are of such a nature that they can be positioned and oriented in any way on the back of the product. Such a printed motif can then be said to be “unsynchronized” in the sense that it does not need to be positioned in a given and precise way along the back of each product. This can, for example, be the case with an irregular pattern or a motif in the form of abstract symbols, the location of which on the back does not need to have a particular geometrical positioning on the product concerned.
There are, however, other types of printed motifs that can be said to be “synchronized” in the sense that they must be placed in a given position on the layer in question so that each individual product is provided with a print that is always in a predetermined position. An example of such a synchronized print can be a motif that is intended to be printed in the middle of the back of the product, that is centred both longitudinally and laterally.
Against the background of the above, it has been found that there is a need for simple, reliable and cost-effective methods and arrangements which have a high level of precision and with which a synchronized print in the form of patterns, characters and other motifs can be provided on an absorbent article. More specifically, the web of material that carries the print in question is to be synchronized in an arrangement for manufacturing the product in question, so that the various work elements that are carried out on the product are carried out in the correct positions in relation to the printed motif.
A previously known way of obtaining such a synchronized printing process is to utilize previously printed reference marks or synchronization marks, that are suitably positioned at regular intervals on the web of material in question. Each synchronization mark can be printed as a small coloured stripe along the edge of the web of material and can be detected electronically by means of an optical detector. Such synchronization marks are then used to control the manufacturing process for the product concerned so that, in its final position, the motif that is to appear on the finished product is always in the intended position on the finished product.
Patent document WO 00/59429 shows an arrangement that utilizes synchronization marks for controlling the positioning of a printed motif on an absorbent product. According to this document, synchronization marks are provided on such sections of the product that are cut away later during the manufacturing process. In this way, temporary synchronization marks are defined that are removed before the product has been completed.
In addition, document WO 99/32384 shows an arrangement for synchronizing two webs of material during the manufacture of absorbent products. One of these webs of material consists of a backing layer that comprises printed motifs, which are then to be synchronized with an additional web of material that comprises an outer layer and an absorbent core. According to WO 99/32384, a stretching of the backing layer is obtained when required, with the object of synchronizing the two webs of material.
The document US 2005/0125180 shows a system arranged for synchronization of different webs of material that are provided with elements, for example in the form of printed motifs, that are positioned at certain given regular intervals. The position of the respective element can then be detected using, for example, a printed synchronization mark.
It can be pointed out that these known systems are based on a printed motif being synchronized by a synchronization mark being detected and by parameters in a processing apparatus being controlled in response to the position of the respective synchronization mark. This is, however, a relatively complex process, and there is a need for methods and arrangements for improved synchronization of printed motifs and similar elements on absorbent articles of various kinds.